The vaccine targeted a flu strain that didn’t look like most of the strains traveling around the Northern Hemisphere during the 2014-2015 flu season, researchers report June 25 in Cell Reports.

Like wearing a hat and glasses or a fake mustache, tiny changes to the strains’ appearance let the virus disguise itself from the immune systems of vaccinated people. The finding explains the U.S. Centers for Disease Control and Prevention’s lackluster estimate of the vaccine’s efficiency: It was only about 20 percent effective at preventing flu-related trips to the doctor.

The new finding will help researchers formulate better vaccines for future flu seasons, the study’s authors conclude.